The trend towards increasingly smaller and lighter four-wheel drive vehicles is leading to four-wheel drive systems, in which the drive power is transmitted, according to specific requirements, from a primary driven front axle to a secondary driven rear axle by means of a clutch. For this purpose an axle differential transmission with engageable drive power of the kind that is of interest herein can be used in the area of the rear axle.
In most cases a clutch assembly is integrated directly in the transmission case between the transmission input shaft, which is disposed on the rear axle along the vehicle for transmitting the driving torque from the front axle to the rear axle, and the input shaft of the differential transmission for a rear axle engagement of the drive. The multiple disk clutch, which is usually designed to cope with the axle torque, is arranged between the crown wheel on the transmission input side and the differential stage. This arrangement makes it possible to disengage the differential with the side shafts from the hypoid drive in the shut down mode. In addition, this arrangement permits a compact design of the rear axle and makes it possible to gain additional design space in the area of the longitudinally arranged transmission input shaft. However, the field of application of the invention is not limited to just an engageably driven rear axle of a vehicle alone. It is also conceivable to assign an axle differential transmission of the type that is of interest here to the front axle of a motor vehicle or to the central axle, provided that the objective is to apply a drive power, which can be engaged on demand, to these axles.
DE 10 2008 037 886 A1 discloses a technical solution for switching off the four-wheel drive at the axle differential transmission of the rear axle of a vehicle. The drive assembly comprises a transfer case, which is assigned to the front axle of the vehicle and which distributes a torque, which is introduced by the drive unit of the motor vehicle, to a first drive train and to a second drive train, wherein the first drive train is permanently drive-connected to the transfer case, in order to transmit a torque to the front axle of the vehicle. In contrast, the second drive train is engageably connected to the transfer case, in order to also transmit a torque to the rear axle of the vehicle, for which purpose a longitudinal drive shaft transmits the torque flow between the transfer case and the rear axle of the vehicle. Within the framework of the drive assembly there are provided first clutch means for coupling and uncoupling the longitudinal drive shaft relative to the drive unit of the motor vehicle as well as second clutch means for coupling and uncoupling the longitudinal drive shaft relative to the rear axle of the vehicle.
To date the second clutch means, which is of interest here and which may be found at the engageably driven axle of a vehicle, has consisted, in principle, of a friction clutch, which is constructed from clutch plates. In this case the friction clutch makes it possible to transmit the torque by means of an axially oriented actuator by pressing the clutch plates against each other.
DE 2008 037 885 A1 discloses an additional solution for the clutch means for an engageably driven vehicle axle in a motor vehicle driven by multiple axles. These clutch means also comprise an externally controllable friction clutch with a clutch input member, which can be driven in such a way that it rotates about an axis of rotation, and with a clutch output member. The clutch means also comprise a differential transmission with an input element and two output elements, which are drive-connected to the input element, wherein this input element of the differential transmission is arranged coaxially to the clutch output member and is driveably connected to the clutch output member, in order to transmit a torque.
In these solutions known from the prior art, the friction clutch in the engageable four-wheel drive is under constant strain. This applies, in particular, to the bearings of the ramp actuators that are often used for actuating the clutch, because in order to transmit the torque, the axial force, which is generated by the actuator, on the clutch plates, has to be maintained. The net result is that there are a plurality of clutch plates that are relatively large in size and an actuator that is dimensioned accordingly.